at the moment I'm running into strange issues with a mix I did for a non-budget project: I did the audio post (mixing, foley, finish) and everything worked as fine as possible (they had some issues with the sound, boom was sometime off-axis...) and the result is ok. When listening to it through speakers or my headphones in the studio everything is fine, but the crew told me when listening to it on several cellphones the dialog was muted, they were unable to hear it.
At first I didn't believe so I opened the file on my iPhone and had no problem with it, so I tried several other cellphones and still no issues. When I finally found an old Nokia and when playing the file the dialog is really, really hard to understand.
My first idea: phasing issue. I double checked the whole mix, listened to it mono but everything is fine. Next idea: maybe a strange filter effect of the cellphones speaker but this doesn't make sense, because it's just the voice that's muted.
As I already said: most of the cellphones I tested work fine, but the crew told me the ones tested by them very often failed and to be honest I have no idea why this happens - never had an issue like that before.